1. Field of the Invention
This invention relates, generally, to pipeline surface preparation systems. More particularly, it relates to machines that travel along the length of a pipeline and remove coating therefrom by the application of water jets at ultra high pressure.
2. Description of the Related Art
The disclosure made in U.S. Pat. No. 5,238,331 to Chapman is believed to be relevant to the present invention because it describes a pipeline surface preparation system that is sufficiently light-in-weight to enable a team of two workers to place it into position around a pipeline in the absence of weight-lifting machinery. A frame surrounds the pipeline and the frame supports wheels that engage the surface of the pipeline and enable the pipeline surface preparation system to travel along the extent thereof. Moreover, the Chapman apparatus employs water jets to strip coating from a pipeline. Water nozzles are circumferentially spaced about the perimeter of the pipeline and limit switches are employed to cause the frame that carries the nozzles to reciprocate along a circumferential path of travel so that hoses connected to the apparatus are not wrapped around the pipeline as the apparatus advances along the length thereof.
The invention disclosed in the Chapman patent therefore solves several problems left unsolved by earlier advances in the field. Earlier devices are so heavy that a crane is needed to lower them into position atop a pipe. The weight of such devices causes the pipe to sag and thus limits the length of pipeline that can be excavated at any one time. When a crane drops one of the early heavy pipeline surface preparation systems onto a pipeline, catastrophic explosions may occur.
However, the art has not heretofore solved all of the outstanding problems associated with pipeline surface preparation systems. One of the outstanding, unsolved problems relates to handling of debris generated by the pipe coating removal process. Old coating commonly includes asbestos and other materials that require special handling. However, the pipeline surface preparation systems of the prior art do not adequately address the debris-handling problem. The conventional wisdom is that Visqueen® plastic or other suitable sheet material should be placed in overlying relation to the ground below the pipeline undergoing reconditioning. Asbestos and other debris is thus collected atop the plastic sheet material as the machine travels along the extent of the pipeline. Workers then carefully fold the plastic sheet material in an attempt to contain the hazardous materials deposited thereatop. The inadequacies of this well-known procedure are readily apparent. Asbestos in small pieces may easily float in the air beyond the reaches of the plastic sheet material and enter the lungs of workers in the vicinity. Asbestos may also enter the lungs of those who attempt to collect it by folding the plastic sheet material into a collection means.
A pipeline surface preparation system that prevents asbestos and other hazardous debris from escaping into the atmosphere and that minimizes total contaminated waste is clearly needed.
The earlier pipeline surface preparation systems also slip from time to time as they travel along a pipe because insufficient engagement is provided between the pipe surface and the rollers or wheels that rotatably engage said surface to cause the pipeline surface preparation system to travel along the pipe. When a pipeline surface preparation system slips, the pipeline can be damaged because the water jets have extended dwell time on the surface. It is very important that the extremely high pressure water jets that are used to strip away pipe coating be applied to the surface with controlled speed and rotation. When a pipeline surface preparation system carrier means slips, exposed pipe is subjected to the full power of the high pressure water jets for an extended time and pipeline damage may occur. A pipeline surface preparation system having improved traction is therefore needed.
A closely related problem is known in the industry as the “hot spots” problem. A “hot spot” is a location on a pipe surface that is subjected to more water pressure than other sections. A hot spot is created whenever a nozzle passes closer to the surface of the pipe in one location than it does in another. Thus, a hot spot may be created by slippage of the transported means as just mentioned, or it may be created by the inherent structural features of the pipeline surface preparation system. The prior art includes a nozzle assembly where a pair of nozzles is mounted to opposite ends of a rotating conduit that is straight in configuration. As a result, the nozzles pass closer to the surface of the pipe in some locations than others, giving rise to the problem of hot spots.
A need therefore exists for a structural design that eliminates hot spots by ensuring that all nozzles are spaced equally from the pipe surface at all times.
It is also important to monitor the air supply and the operation of all water-emitting nozzles because any failed movement will direct a high pressure water stream to a single spot and thereby damage the pipe. Automatic monitoring means should therefore be connected to the air supply, each nozzle, and other movable parts. A means should be provided for sending a “shut-down” signal to the ultra high pressure system when any required motion fails.
Yet another outstanding problem relates to the debris created by the stripping action of the water jets. The known pipeline surface preparation systems produce debris in large particle size. The art has addressed this problem by providing an external shredding means to reduce the debris particles to a more manageable size. The price of an external shredder increases the cost of the system, the time required to operate the external shredder decreases productivity, and the operation of the shredder could potentially add to environmental concerns with hazardous wastes.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the art how the known pipeline surface preparation systems could be improved.